1. Field of the Invention
The invention relates to the drying of gases. More particularly, it relates to a membrane process and system for the enhanced drying of air and other gases.
2. Description of the Prior Art
Permeable membranes capable of selectively permeating a component of a gas mixture are considered as a convenient and potentially highly advantageous means for accomplishing desirable gas separations. To realize this potential in practical commercial operations, membrane systems must be capable of achieving a desired degree of processing efficiency. As water vapor and other condensable gases, such as carbon dioxide, are known to be highly permeable in many membrane materials, the drying of air or other gases is a significant membrane application, and one in which enhanced drying effectiveness is needed to satisfy the ever-increasing requirements of the art.
It is common practice to use membrane materials in the form of a multiplicity of small hollow fibers, arranged so that pressurized feed gas is exposed to a large membrane surface area, with the component thereof that is selectively permeated through the hollow fibers being removed as lower pressure permeable gas. The non-permeable gas is received at essentially the permeation pressure level. In such operations, the feed gas is commonly passed over the outside, or shell side, of the hollow fibers, while the permeate gas is removed from the fiber bores, this approach being referred to as an "outside-in" flow pattern. It is also possible to provide for the passage of the pressurized feed gas to the bores of the fibers, with the permeate gas being removed from the shell side space outside the fibers, i.e. in an "inside-out" flow pattern. Both of these patterns have been employed in the art in commercial membrane permeation processes.
If the high pressure feed gas and the lower pressure permeate gas separated by the membrane were in a stagnant or non-flowing condition, the partial pressure of the more permeable component, e.g. water or an impurity in the feed gas being treated, would approach a common equilibrium value on both sides of the membrane, and no further permeation of the more permeable component would occur. In practical operations, therefore, it is necessary that the feed gas be passed along the hollow fiber membrane, on either the bore side or the shell side, so that the partial pressure of the more permeable component can continue to decrease and separation thereof from the feed gas can continue to occur. This requires also that the lower pressure permeate stream have a progressively lower partial pressure for the more readily permeable component. It is well known in the art that such conditions can be advantageously achieved by disposing the permeable membranes in modules or columns arranged so that the higher pressure feed stream and the lower pressure permeate stream are caused to flow in opposite directions. Such operations are referred to generally as countercurrent flow operations.
In drying operations in which water or any impurity of a highly permeable nature is to be separated from air or other feed gas stream, it is also common practice to pass a dry or impurity-free gas on the lower pressure permeate side of the membrane as a purge gas to facilitate the maintaining of a partial pressure differential, and resulting permeation driving force, across the membrane.
Membrane modules employing countercurrent flow conditions and a purge gas stream have been used for drying small quantities of air for use in laboratory or analytical instruments. Such membrane module dryers are columns designed with four gas ports, i.e. (1) an inlet feed gas port, (2) an outlet non-permeate product gas port, (3) an input port for purge gas, and (4) an output port for the purge gas and the permeate gas. Such dryers employ homogeneous polymeric hollow fibers, with the fibers having a sufficient thickness to support the desired pressure difference across the membrane. The permeation rate for such membranes expressed by the permeability/thickness (P/t) ratio, is rather small, even with respect to high permeability gases, because of the required thickness of such homogeneous material membranes. As a result, such homogeneous hollow fiber membranes are not well suited for large-scale commercial gas drying applications.
Membrane technology is also known in the art, for gas drying and other fluid separations, wherein membrane configurations other than those based on a homogeneous or uniform density of a single membrane material are employed. Thus, composite and asymmetric hollow fiber membranes are available for desirable fluid separations. Composite membranes comprise a thin separation layer or coating of a suitable permeable membrane material superimposed on a porous substrate. The thin separation layer determines the separation characteristics of the composite structure, with the porous substrate providing physical support for the separation layer. Asymmetric membranes, on the other hand, are composed of a single permeable membrane material having, in distinction from homogeneous membranes, a thin, dense semipermeable skin region that determines the separation characteristics of the membrane, and a less dense, porous, generally non-selective support region that serves, as does the porous substrate of composites, to preclude the collapse of the thin skin region under pressure. Both types of non-homogeneous hollow fiber membranes, i.e. composites and asymmetrics, exhibit relatively high permeability/thickness ratios compared to those of homogeneous membranes, particular with respect to the permeability of condensable gases.
For use in practical commercial operations, membrane structures of the types indicated above are commonly employed in membrane assemblies or bundles that are typically positioned within enclosures to form membrane modules, the principal element of an overall membrane system. Such a membrane system commonly comprises a membrane module, or a number of such modules, arranged for either parallel or series operation.
Using non-homogeneous hollow fibers for gas separation applications, the high pressure feed gas stream frequently is applied to the side of the membrane hollow fiber on which the separation portion of the membrane is positioned, whether this be on the inside or the outside of the hollow fiber. The gases that permeate the separation layer or skin thus pass into the porous substrate portion of the membrane and are removed from the non-separation side of the membrane structure.
Hollow fiber membrane modules have commonly been fabricated so that, in the local vicinity of each individual hollow fiber, the flow patterns approximate cross-flow, even though the global flow arrangement might appear to be countercurrent. In cross-flow operation, the flow direction of permeate gas on the permeate side of the membrane is at right angles to the flow of feed gas on the feed side of the membrane. For example, when the passage of feed gas is on the outside of the hollow fiber membranes, the flow direction of permeate gas in the bores of the fibers is generally at right angles to the flow of feed gas over the external surface of the hollow fibers. Likewise, in the inside-out approach in which the feed gas is passed through the bores of the hollow fibers, the permeate gas generally passes from the surface of the hollow fibers in a direction generally at right angles to the direction of the flow of feed gas within the bores of the hollow fibers and then, within the outer shell, in the direction of the outlet means for the permeate gas. Such cross-flow type of flow pattern is to be distinguished from a countercurrent flow type of flow pattern. In such countercurrent flow pattern, the feed gas or the permeate gas, depending on whether inside-out or outside-in operation is desired, is caused to pass in countercurrent flow along the outside surface of the hollow fibers parallel to the flow direction of permeate gas or feed gas in the bores of the hollow fibers. The feed gas on the outside of the hollow fiber bundle, for example, is caused to flow parallel to rather than at right angle to the central axis of the hollow fiber.
In membrane drying operations of the type described above, there is a tendency to encounter a concentration polarization across the substrate portion of the membrane and to operate under cross-flow permeation characteristics when using composite or asymmetric membranes as opposed to a dense, homogeneous membrane fiber. Where such a concentration polarization occurs to a significant extent across the substrate portion of the membrane, i.e., a concentration gradient exists across said substrate portion, the driving force across the thin separation layer of the composite membrane or across the thin skin portion of an asymmetric membrane is thereby decreased. In the absence of such concentration polarization, the pressure differential between the feed gas and the permeate gas streams on opposite sides of the membrane can be effectively utilized to facilitate the desired selective permeation of water from feed air or other desired drying operation. In this regard, it is important to appreciate that even if the concentration of permeating component were the same on both sides of the substrate, i.e. 0% concentration polarization, which condition is also sometimes referred to as "perfect radial mixing," but the flow patterns across the membrane bundle are of the cross-flow type, the overall permeation result would be consistent with the conventional mathematical model for cross-flow permeation. Similarly, if the flow patterns of the membrane bundles were arranged for countercurrent operation, but the fiber design morphology were such that a concentration polarization fully formed across the substrate, i.e. 100% concentration polarization, the overall performance of the membrane would again be consistent with the model for cross-flow permeation and not for countercurrent permeation. Those skilled in the art will appreciate that countercurrent permeation operations, in which a significant degree of radial mixing is achieved, are generally desired and provide higher permeation levels than cross-flow permeation operations as confirmed by the mathematical models for the two types of operation.
When membrane drying operations are carried out using a homogeneous dense fiber membrane as indicated above, a significant level of the desired countercurrency is achieved, and such dense fiber membranes can generally be employed either with the use of a purge gas on the permeate side, as indicated above, or without such purge gas. In the latter case, good drying requires operation at a relatively high stage cut, i.e. a considerable amount of the gas being dried must co-permeate with the water in order to flush said water from the membrane system in the waste stream. Such operation is not suitable when high product recovery is required.
It will be appreciated that the membrane thickness of a dense fiber is also its wall thickness, which is very large in comparison to the skin portion of an asymmetric membrane or to the separation layer of a composite membrane. For a dense fiber, it is necessary to have a large wall thickness to achieve a significant pressure capability. Thus, dense fibers have a very low permeation rate and require the use of a very large surface area to achieve adequate drying in commercial operations. This tends to be a critical disadvantage in commercial applications due to the large costs associated with the providing of such membrane area. As noted above by contrast, asymmetric or composite membranes have very thin membrane separation layers, with the more porous support portion thereof providing mechanical strength and support for the very thin portion that determines the separation characteristics of the membrane. Much less surface area is required, therefore, for asymmetric or composite membranes than for dense, homogeneous membranes.
While dense membranes do not tend to experience concentration polarization across the surface thereof, thus enabling such membranes to exhibit countercurrent permeation, both asymmetric and composite membranes are subject to concentration polarization and have tended to exhibit cross-flow permeation flux (i.e. permeation/time) characteristics in practical applications thereof. Because of the inherently improved permeability obtainable by the use of asymmetric or composite membranes rather than dense membranes, it would be desirable in the art to further improve asymmetric and composite membrane performance to facilitate the achieving of the benefits of membrane drying and other separation operations in practical commercial operations.
Mathematical modeling for the analysis of membrane performance is illustrated in "Gas Separation by Permeation, Part I. Calculation Methods and Parametric Analysis" by C. Y. Pan and H. W. Habgood, The Canadian Journal of Chemical Engineering, Vol. 56, April, 1978, pp. 197-209. Utilizing such mathematical modeling analysis techniques, C. Y. Pan concluded, based on an analysis of asymmetric membranes, that said membranes always give rise to cross-flow type of permeation operation, regardless of the flow pattern and the direction of flow of the feed and permeate streams. Such conclusions, and the mathematical basis therefor, are disclosed in "Gas Separation by Permeators with High Flux Asymmetric Membranes" by C. Y. Pan in the AIChE Journal, Vol. 29, No. 4, July, 1983, pp. 545-552.
On the basis of such analysis, it was commonly concluded that air and other gas separation operations were necessarily operable in a cross-flow manner and that, to enhance such operations, the membrane bundle design should be a design serving to enhance such cross-flow permeation operation. Accordingly, many membrane bundles are provided with a multiplicity of holes along the longitudinal length of the bundles to facilitate the carrying out of cross-flow permeation patterns. As mentioned above, even if the morphology of the substrate were such as to preclude concentration polarization under such circumstances, the overall membrane performance achieved using such bundle design would be that consistent with the mathematical modeling for cross-flow permeation.
It has more recently been observed in the art that, contrary to such earlier prior art expectations, many membranes exhibit a significant degree of countercurrency in operation, with less concentration polarization than would have previously been predicted. Thus, M. Sidhoum, W. Sengapta and K. K. Sirkar, in "Asymmetric Cellulose Acetate Hollow Fibers: Studies in Gas Permeation", AIChE Journal, Vol. 34, No. 3, March 1988, pp. 417-425, refer to such earlier mathematical modeling and the indicated creation of a cross-flow permeation pattern for asymmetric membranes (as contrasted to the pattern for the less desirable symmetric or homogeneous membranes). The authors reported that the behavioral pattern of such membranes supposedly characterized by cross-flow permeation patterns did not fully follow the modeling for cross-flow operation, but better followed the homogeneous membrane model that is consistent with countercurrent flow behavior. Contrary to prior expectations, therefore, many asymmetric and composite membranes do, in fact, possess a significant degree of countercurrency. It is highly desirable in the art, in light of this evolving understanding, to develop improved membrane drying and other gas separation processes and systems to enhance countercurrent permeation.
It is an object of the invention, therefore, to provide an improved process and system for membrane drying and other separation applications.
It is another object of the invention to provide a composite or asymmetric hollow fiber membrane separation process and system possessing an enhanced degree of drying capability.
It is a further object of the invention, to provide a membrane process and system for minimizing the membrane surface area and product permeation loss necessary to achieve a desired level of drying or like separation.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.